The seniority rule refers to a custom in the United States Congress that grants committee chairmanship to a majority party member who has served on the committee for the longest amount of time. Service time on the committee must be unbroken for the seniority rule to be applied.
The most senior member of the committee that belongs to a minority party, on the other hand, is typically designated as the ranking committee member. This committee member has the ability to hire and fire staff members for the committee's minority party. This mirrors the power of the majority party committee chairman, who is able to hire and fire a larger number of the committee's staff.